Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

The difference between the two may be explained by a subtle variation in the brain's structure, according to a study published in the Journal of Neuroscience.

About 50 percent of people are born with a prominent fold in the brain matter that lies just behind the forehead, explained study co-author author Jon Simons, a researcher at the University of Cambridge in the UK. Simons and his colleagues found that these are the people who seem record very accurate versions of what has happened to and around them.

In contrast, people with a less pronounced fold, or a non-existent one, seem to have problems distinguishing between what they actually experienced and what they might have imagined or heard about, Simons said.

Scientists long ago mapped out the hills and valleys of the brain. One puzzling part of that cranial geography is the significance of a missing fold in the outer part of the brain, or cortex. That fold is one of the last structures to develop in a growing fetus, Simons explained.

Simons and his colleagues suspected that this fold might somehow be involved in cataloguing memories as either real or imagined. So they designed an experiment that would distinguish between people with exacting and muddled memories.

The first step was to pore through thousands of brain scans from normal healthy people, looking for ones that showed either a prominent fold, or ones in which the fold was missing in either the right or left hemisphere.

The researchers chose 53 study volunteers, which were almost equally divided between ones with a prominent fold, ones with some folding in the left hemisphere, ones with some folding in the right hemisphere and the rest with no fold whatsoever. Because the brain fold straddles the central area of the brain, some folding may be on the left and some on the right, or both.

All 53 were shown words on the screen that often come as well-recognized pairings, such as bacon and eggs, Laurel and Hardy, Abbott and Costello. Sometimes both words were shown. Sometimes one word from a pair was shown, followed by a question mark. Then the study volunteers either heard the word pairs read by a researcher or were asked to read the words themselves aloud.

Later on, the volunteers were asked whether they actually saw both words on the screen or just one word with a question mark. They were also asked whether they read the words aloud themselves or heard someone reading the words to them. When the researchers scored the memory test they found the volunteers with no fold scored the worst, while those with prominent folds scored the best.

The new research is very intriguing, said Paul Thompson, a professor of neurology at the University of California, Los Angeles. Scientists have long known about this variation, but no one really knew what its consequences were.

Ultimately, Thompson said, the study might offer some insight into diseases like schizophrenia, in which people have trouble distinguishing between what is real and what is a hallucination.

It might also have an impact on how judges and juries perceive eye-witness testimony, Simons said. You could imagine that people without the fold “might witness a crime and then talk to someone else,” he explained. “Or they might read a newspaper report about the crime and then misremember what they actually saw.”

The tricky thing about this kind of memory issue is that people generally don’t recognize they have it, Simon said. They think their memories are every bit as accurate as everyone else’s.

Perhaps one day some enterprising defense lawyer will ask for brain scans of prosecution witnesses to see if they have accurate memories.

Linda Carroll is a regular contributor to msnbc.com and TODAY.com. She is co-author of the new book "The Concussion Crisis: Anatomy of a Silent Epidemic”